Methods for the production of nitrate esters are known and are in commercial use. The known methods utilize strong nitric acid or admixtures of nitric acid with sulfuric acid and/or acetic acid to nitrate the appropriate hydroxy-containing organic substance. Typically, such mixed acid systems have been used because they provide enhanced yields and chemical efficiency. Mixed acid nitration media are often used to produce high NO.sub.2.sup.+ concentrations in order to achieve complete nitration. High NO.sub.2.sup.+ concentrations are reported to occur when essentially equal molar amounts of nitric acid and sulfuric acid are utilized. As reported by Urbansky, the production of unwanted by-products increases as the amount of secondary acid is reduced or eliminated. Urbansky further reports that the yield of desired product decreases as the amount of nitric acid is reduced. The resulting nitrate ester produced from the above-described processes is usually insoluble in the spent acid solution and thus forms an immiscible layer of product and/or a solid precipitate. The product and the spent acid solution are both relatively stable after separation.
However, some commercially desirable nitrate esters are soluble in the spent acid solution. When the desired nitrated ester product is partially soluble in the spent acid solution, very low yields of the nitrate ester are obtained and the spent acid solution becomes very unstable, usually "fuming off" within a few minutes. The unstable spent acid solution admixture and/or product is due to the presence of mixtures of organic materials and oxidizing acid.
In the past, to alleviate the problem of producing the nitrate esters which prove to be soluble in the spent acid solution, an inert organic solvent has been added to the nitric acid or mixed acid medium in the nitration of the hydroxy-containing organic substance. An example of a process known in the chemical industry includes the method disclosed in European Patent Application No. 843037524. The chemistry of the reaction in that method necessitates the use of an organic solvent with the acid in the nitration medium. The use of an organic solvent with the acids in the nitrating medium causes the partially soluble nitrate esters to be extracted into the organic phase, which can then be separated and recovered, resulting in an increased yield of nitrate ester product. These organic solvents are chosen and used so as to be effective to remove the heretofore partially soluble nitrate esters from the acid phase, enhancing the recovery of the desired nitrate ester. Furthermore, these organic solvents are used in an amount effective to cause the complete extraction of nitrate esters into the organic phase. The organic solvent used must be insoluble in water, inert toward nitration, and volatile so that the product may be recovered. These restrictions limit the choice of solvents which can be utilized to halogenated aliphatic compounds, preferably chloroform and/or methylene chloride. Using these solvents in the nitration mixture will provide very high yields of clean, stable products and stable spent acid solutions. However, as set forth above, the only solvents which are inert to nitration, insoluble in water, and volatile are the halogenated aliphatic hydrocarbons, which are considered toxic or hazardous substances in many geographic locations. The use of these solvents requires special handling and treatment of all effluents and waste materials which come into contact with the solvent. Also, when such organic solvents are used in the nitration step, recovery of the final product requires the removal of the spent acid phase, wherein the spent acids are recovered either separately or as a mixed stream, followed by the separate step of neutralization of the product/solvent solution. Thereafter, the solvent must be removed in order to recover the nitrate ester product. Therefore, the use of such solvents substantially increases the cost of production of those nitrate esters which are somewhat soluble in the spent acid mixture. Furthermore, at the present time, there is some concern about the future availability of halogenated aliphatic hydrocarbon solvents such as methylene chloride.
Thus, there has been a need to provide a process by which high yields of water- or acid-soluble nitrate esters may be prepared without the use of organic solvents in the nitration reaction or the production of unstable products or by-products.